NEW YORK (CBSNewYork/AP) — Prosecutors said Monday that Osama bin Laden’s son-in-law should be sentenced to life in prison for his role as al Qaeda’s spokesman after the Sept. 11 attacks, which involved he made “bone-chilling threats” against America.

In court papers filed Monday, the government said a life sentence for Sulaiman Abu Ghaith, whose words were “inciting, murderous, and hateful,” would send an important deterrent message to would-be terrorists.

“Terrorism is a crime with high recidivism rates and rehabilitation is notoriously difficult,” prosecutors said.

The papers were filed in U.S. District Court in Manhattan a month before a sentencing scheduled for Sept. 23. Abu Ghaith’s defense lawyer, Stanley Cohen, had urged Judge Lewis A. Kaplan earlier this month to sentence his client to 15 years in prison, saying it was sufficient.

Abu Ghaith was convicted in March of charges that included conspiracy to kill Americans and providing support to al-Qaida. Abu Ghaith is a Kuwaiti-born imam who married bin Laden’s eldest daughter about five years ago. He is the highest-ranking al-Qaida figure brought to trial on U.S. soil since the 2001 attacks.

In their papers, prosecutors said Abu Ghaith “proudly sat at bin Laden’s right hand” after the Sept. 11 attacks, repeatedly participating in videos aimed at recruiting more people to al-Qaida.

In his trial’s most dramatic testimony, Abu Ghaith described being summoned to a dark Afghanistan cave within hours of the destruction of the World Trade Center’s Twin Towers to confer with bin Laden, who told him: “We are the ones who did it.”

Abu Ghaith testified that a worried bin Laden asked him how America would respond.

“America, if it was proven that you were the one who did this, will not settle until it accomplishes two things: to kill you and topple the state of the Taliban,” Abu Ghaith said he replied.

In his papers, Cohen said his client had no prior knowledge or role in the planning or execution of the Sept. 11 attacks or any other bombing or terrorist activity.

“His role was confined to speaking, after the attacks had already happened,” he wrote.

Cohen said a sentence of 15 years would really be 25 years, since Abu Ghaith spent a decade in prison in Iran, much of it under extreme conditions which included abuse and torture.

But prosecutors disputed those claims, saying Abu Ghaith formed a community with other Arab detainees and even married two women while incarcerated, fathering multiple children along the way.